Hi Slowflyingmonkey,
Let's break your question down a bit.
You asked how decks are populated on apps.
Are you asking how a Deck is shuffled and dealt?
Let's look at shuffling a physical Deck first.
Shuffling is a procedure used to randomise a Deck of Cards to provide an element of chance. The number of shuffles required to produce an acceptable level of randomness depends on the type of shuffle and the interpretation or perception of what constitutes an acceptable shuffle.
Randomness can be defined as every Card being equally likely to be in any position. If a Deck is not random, it means the Cards can be predicted from their original position at a frequency better than random guessing.
Now let's look at a virtual/online tarot software shuffle of a Deck:
A computer algorithm can generate a random permutation of the Cards. Simply put, the program can repeatedly swap the position of two different Cards in a Deck imitating a physical shuffle.
It is said that a cartomancy (or playing) 52-Card Deck is considered random after 7 riffle shuffles compared to about 2,500 overhand shuffles. A riffle shuffle is when the Deck is divided into two halves, taking one half in the left hand, and the other in the right. Cards are then alternatively interleaved from the left and right hands to make one pile.
An overhand shuffle places the Deck in one hand, then, slides a small number of Cards from the top into the other hand until all Cards are in the other hand.
So in some respects, using online or virtual Tarot programs could actually produce a more random shuffle than a physical shuffle of a Deck. I mean, who is going to physically shuffle a Deck 2,500 times, right?
Tarot software can also use a computer algorithm to create a starting point (Seed) for the shuffle. The Seed is the numerical value of the word or phrase entered into the Seed Shuffle field. Even though the word or phrase could be a variable, the result of using the same Seed on multiple occasions may produce different results as each previous shuffling is independent. It might be that some of the online virtual tarot websites use seeds based on what you say when you are asked to enter information about your question.
I know that Orphalese Tarot software, for example, offers multiple ways of shuffling the Decks (including seeds).
Nothing really compares to holding the physical Deck in your hand but online Tarot and Tarot software programs also have an important place. For example, a program like Orphalese offers the ability to easily replicate a Reading created with a physical Deck for the purpose of saving an electronic copy of that Reading for later review and reflection. (Or, when you are reading for another person and want to send them an email of Card images, positions in the Spread and Descriptions/Notes.)